Cloning and characterization of the grass carp (Ctenopharyngodon idella) Toll-like receptor 22 gene, a fish-specific gene

Abstract

Toll-like receptor 22 (TLR22) is a fish-specific TLR which recognizes double-strand (ds) RNA and participates in the innate immune response through the Toll-IL-1R homology domain-containing adaptor protein 1 (TICAM-1). To further investigate how the innate immune system of teleosts responds to viral infections, we cloned the full-length cDNA sequence of grass carp (Ctenopharyngodon idella) TLR22 (CiTLR22). The complete cDNA sequence of CiTLR22 was 3647 bp and encodes a polypeptide of 954 amino acids. Analysis of the deduced amino acid sequence indicated that CiTLR22 has typical structural features of proteins belonging to the TLR family. These included 17 LRR domains (residues 88-634) and one C-terminal LRR domain (LRR-CT, residues 694-745) in the extracellular region, and a TIR domain (residues 801-944) in the cytoplasmic region. Comparison with homologous proteins showed that the deduced CiTLR22 has the highest sequence identity to common carp TLR22 (82.9%). Genomic DNA of CiTLR22 was obtained by long-distance (Ld) PCR and structure analysis revealed that the CiTLR22 gene is encoded by uninterrupted exons. Reverse transcriptase-PCR (RT-PCR) revealed that CiTLR22 is a non-maternal gene. It is prominently expressed in immune relevant tissues such as spleen and head kidney. Quantitative RT-PCR analysis showed that CiTLR22 transcripts were upregulated significantly in immune relevant tissues and blood following grass carp reovirus (GCRV) infection. In the whole genomic sequence, nine single nucleotide polymorphisms (SNPs) were detected. Seven of them were sited in the coding region, and the other two located in the 5' and 3' untranslated region (UTR) respectively. None of the SNPs was associated with the resistance of grass carp to GCRV. These results suggested a role for CiTLR22 in mediating immune protection against viral infection in grass carp.